Integrated circuit pressure transducers have been developed by utilizing equal resistive value piezoresistive elements on a single crystal silicon diaphragm. The operation of these devices is such that a change in pressure develops a strain in the silicon diaphragm which is effectively magnified by the diaphragm to vary the resistance of the piezoresistive elements. The piezoresistive elements typically include p type material which is diffused into an n type silicon diaphragm by standard phtoresist and diffusion techniques. By diffusing the piezoresistive elements orthogonally into the silicon diaphragm, a differential pressure across the silicon diaphragm causes the resistance of one piezoresistive element to increase and the resistance of the other piezoresistive element to decrease by an equal amount, in other words, a piezoresistive element subject to transverse strain changes its resistances by an amount equal in magnitude but opposite in sign to the change in resistance resulting from the application of the same strain in the longitudinal direction. These semiconductor pressure transducers are useful for observing and measuring very small pressure changes. The resistance of the piezoresistive elements changes with a change in pressure as a result of a change in the bulk resistivity of the piezoresistive material rather than as a result of a change of geometry, as in the case with wire or foil strain gages. This change of bulk resistivity is due to the effect of strain upon the asymmetric conduction band energy relative to valence band energy.
The output from the semiconductor pressure transducer is preferably digital rather than analog. A digital output is not noise or distance limited when transmitting the transducer output signals to a distant location. Additionally, analog to digital conversion errors are eliminated and digital integrated circuit techniques can be used to fabricate the entire transducer. In addition to reduced cost and increased reliability the digital output enables the transducer to be more readily temperature compensated.
Accordingly, one object of this invention is to provide a semiconductor digital pressure transducer system.
Another object of this invention is to provide an improved digital piezoresistive pressure transducer.
Still another object of this invention is to provide a digital piezoresistive pressure transducer that is economical to fabricate and reliable in operation.